This invention relates generally to electrical power generating systems, and more specifically relates to power generating facilities which include a plurality of turbine-generator sets.
Industrial gas turbine-generator sets, such as are used in modern electrical power generating facilities, require some type of auxiliary input power for enabling start-up from a rest (or 0 rpm) condition, to that rotational speed where the gas turbine produces sufficient power to sustain its own operation --and then accelerate to full operating speed. The start-up period for a typical such set is of the order of 10 minutes, and e.g., for a turbine having a power rating of 50,000 kilowats, the starting power is about 1200 hp. It has been well-known in the past to develop this auxiliary starting power by using an induction motor, which drives the set through a torque converter.
More recently, however, it has come to be recognized that the power required for start-up purposes may be produced by operating the generator member of the set as a synchronous motor. Reference may be usefully had in this connection, to U.S. Pat. No. 3,764,815 -- which generally discloses a technique of this type being utilized for starting up gas turbines, pump storage units and the like.
In order to motorize the generator member of the set in the manner indicated, a frequency converter is required. Such system basically functions to convert the fixed AC 60 Hz grid frequency to DC; and then the DC is converted to alternating current at a frequency from zero to an upper design limit, as determined by a variable frequency control system. The main components of such system are the rectifiers (such as thyristors), and the control system for regulating the firing rate of the thyristors -- or the output rate of such other elements as may be used. As is well-known to those skilled in the present art, these frequency conversion systems are complex and very costly.
In addition, to the start-up requirement which has thus far been discussed, a particular problem arises when the operating turbine-generator set is to be shut down. In particular, as is known to those familiar with the present art, it is necessary during such shut-down, to rotate the shaft at a low speed for a sustained period, in order to maintain the uniform temperature distribution until the immense quantities of residual heat have been dissipated. The required shaft-turning period can be as long as 30 hours or more, depending upon the specifics of the set. The shaft-turning power thus required for a typical 50,000 KW set is, however, only of the order of 5 hp at about 60 rpm.
The motorized generator start-up system, in addition to effecting its start-up function could, of course, also be used to drive the shaft at low speed for the cool-down cycle. Such arrangement would, in particular, be prefectly acceptable in an installation where but a single turbine-generator set is present.
In power plant installations containing a plurality of turbine-generator sets, however, a more complex and previously unmanageable problem is presented. In particular, a single motorized generator starting system can be used for starting each unit one at a time. However, if this starting system is also to be used as a low speed turning system for shaft cooling, then the number of units to be operated from one control system is limited to two sets. The reason for this limitation is due to the shaft-turning function and not the starting function, in that the starting of multiple units can be effected serially in time; but it may be necessary to provide a capability of having all units in an installation on a low speed shaft turning configuration, simultaneously.
In addition, the start-up system must be sufficiently flexible to allow interrupting the low speed turning function of the multiple units for start-up of any one of the turbine-generator sets.
It may be further noted, that where the number of turbine-generator sets are no more than two, a single motorized generator starting system can be used to start each unit serially in time. Should both units in this instance be shut down at the same time then it would obviously be necessary for both units to be operated in parallel during the cooling period. With two units, this can be accomplished by connecting the start-up system to drive the first set while still permitting the second set to coast down. A shaft position indicator on the first set can signal its pole position and speed to a synchronizing device, which simultaneously monitors the output of the start-up system, and then connects the second in parallel with the first upon synchronization of frequency and phase being attained. Thereafter both units can be operated in parallel.
The aforementioned scheme however, is complicated and may not be employed where more than two units are present. This may be appreciated if one considers the case where three turbine-generator sets have been shut down together, and each is decelerating at its own, slightly different rate. The signal for the pole position of generator B could be monitored and unit A brought to synchronization with unit B, but there is no way to assure that unit C could be similarly treated. Were the coast-down characteristics of the units precisely identical, then unit C could be synchronized by disconnecting units A and B while they were rotating in synchronization, and using the start-up control to bring unit C also into phase. However, the characteristics of the units are known not to be that closely related.
In accordance with the foregoing, it may be regarded as an object of the present invention, to provide a relatively simple system, which enables a single motorized generator start-up system, to start up the turbine-generator sets of an installation including a plurality of such sets; and which, further, enables transfer of any number of such sets to low speed shaft-turning configurations, without a requirement for additional variable frequency power supplies and attendant circuitry and components.
If is a further object of the present invention, to provide a system for use in power installations of the type including a plurality of turbine-generator sets, wherein any number of the turbine-generator sets may be placed and maintained at a low speed configuration for cooling or other purposes; and wherein such result is achieved by the use of subsystems of low cost and complexity.